1. Technical Field of the Invention
The present invention relates generally to spark plugs for internal combustion engines.
More particularly, the invention relates to a compact spark plug which includes a metal shell having an M10 or M12 threaded portion and is highly gas tight.
2. Description of the Related Art
Conventional spark plugs for use in internal combustion engines generally include a tubular metal shell, an insulator, a center electrode, and a ground electrode.
The tubular metal shell has a length; it also has a first end and a second end that are opposite to each other in the lengthwise direction of the metal shell. The metal shell includes a threaded portion on the outer periphery thereof for fitting the spark plug into a combustion chamber of an engine.
The insulator has a center bore formed therethrough; it is fixed in the metal shell such that an end thereof protrudes from the first end of the metal shell.
The center electrode is so secured in the center bore of the insulator that an end thereof protrudes from the end of the insulator.
The ground electrode has a based end joined to the first end of the metal shell and a tip portion that faces the end of the center electrode in the lengthwise direction of the metal shell through a spark gap formed therebetween.
In such a spark plug as described above, in order to form a hermetic seal between the metal shell and the insular at the second end of the metal shell, sealing members are provided in a gap formed between the inner surface of the metal shell and the outer surface of the insulator in proximity of the second end of the metal shell. The sealing members include two metal rings and powdered talc, which are embedded in the gap such that the talc is interposed between the two metal rings in the lengthwise direction of the metal shell.
Further, in order to form the hermetic seal, the metal shell is crimped at the second end thereof, thus forming a crimped portion of the metal shell. The crimped portion of the metal shell exerts a constricting force on the sealing members, whereby the hermetic seal between the insulator and the metal shell is achieved and the insulator is fixed to the metal shell.
In recent years, the demand for higher power output of internal combustion engines has required increasing the sizes of intake and exhaust valves for the engine and securing a water jacket for cooling of the engine. This results in a decreased space available for installing a spark plug in the engine, thus requiring the spark plug to have a compact (more specifically, slenderized) structure.
Specifically, the threaded portion of the metal shell in a spark plug had a size of M14 as specified in JIS (Japanese Industrial Standards) in the past. For example, Japanese Patent First publication No. 2001-307858, an English equivalent of which is U.S. Pat. No. 6,707,237, discloses a spark plug that includes such a M14 threaded portion of the metal shell. However, the threaded portion of the metal shell is now required to have a size of M10 or M12 as specified in JIS.
Further, it is also required to reduce the size of a polygonal prism-shaped portion of the metal shell, to which torque is applied by a wrench when the spark plug is installed in the engine. More specifically, to make the spark plug compact, it is required to reduce the width between any two opposite side surfaces of the polygonal prism-shaped portion of the metal shell.
However, in the meantime, such reductions in the size of the threaded portion of the metal shell and the width of the polygonal prism-shaped portion of the same may cause a problem in which the metal shell cannot be rigidly crimped at the second end thereof.
This is because when the spark plug is made compact, the radial thickness of the metal shell is accordingly reduced. Consequently, the wall thickness of the crimped portion of the metal shell is also reduced, thus resulting in a decrease in the rigidity of the crimped portion of the metal shell.
As a result, the crimped portion of the metal shell cannot exert a large constricting force on the sealing members in the gap between the outer surface of the insulator and the inner surface of the metal shell, thus making it impossible to secure a highly gas tight spark plug.
To solve such a problem, one may consider, instead of reducing the radial thickness of the metal shell, reducing the radial thickness of the insulator for making the spark plug compact.
However, in the meantime, such reduction in the radial thickness of the insulator, which electrically isolates the center electrode from the metal shell, may cause a flashover of the insulator.
Accordingly, it is impossible to reduce the radial thickness of the insulator for the purpose of making the spark plug compact.